My RE told me to stop around 10.5 weeks, because he said that's about the time the placenta begins to take over, and you're pretty much out of the danger zone. So I went from two suppositories a day, to one a day for like 4-5 days at and then stopped them at around 11 weeks. (I never had progesterone issues though-just a precaution since it was IVF). I would read up on the placenta thing though, I think when that's established, you have no real need of the supplements.
Wife (34) of 13 years to DH (36), Mom to Logan
(10), and Tristan and Cael,
the wee ones, born 3/18/11, and little baby Silas
born 1/22/13. TTC #5!